The KEYNOTE-054 trial investigated the use of Keytruda as a monotherapy given after surgery to people at high risk of seeing their cancer recur, and showed that the drug extended recurrence-free survival (RFS) by 43% compared to placebo.

The trial - partnered with the European Organisation for Research and Treatment of Cancer (EORTC) - will be continued to evaluate other endpoints, including overall survival, and the data will be submitted to regulatory authorities said Merck (known as MSD outside the US and Canada) in a statement.

Keytruda isn’t the first PD-1 inhibitor to show activity in this setting. At last year’s European Society for Medical Oncology (ESMO) congress, Bristol-Myers Squibb’s Opdivo (nivolumab) was found to be more effective than Yervoy (ipilimumab) - BMS’ CTLA4 inhibitor which is already approved for adjuvant treatment of melanoma post-surgery - in the CheckMate-238 phase III trial and has already been approved for adjuvant use in the US and Europe.

Two positive trials make it likely adjuvant anti-PD-1 will become standard of adjuvant care for high-risk melanoma, rather than Yervoy which is associated with much more severe side effects.

“This result shows a significant advancement for patients that could potentially change the way melanoma is treated in the future,” said Alexander Eggermont, director general at the Gustave Roussy Cancer Institute in Paris, who chaired the trial.

For Merck the adjuvant indication could help Keytruda close the gap with Opdivo, which remains the biggest-selling PD-1 inhibitor despite a setback in the big first-line non-small cell lung cancer indication. Both drugs are approved for multiple indications, but Opdivo is still out-selling Keytruda, with the two drugs making sales of $3.6bn and $2.5bn respectively in the first nine months of 2017.